1. Field of the Invention
The present invention relates to series-resonant-loaded inverters, particularly as used for powering gas discharge lamps.
2. Description of Prior Art
In an inverter where a gas discharge lamp load is parallel-connected across the tank capacitor of a high-Q LC circuit that is resonantly series-excited by a high-frequency voltage output of the inverter, it is necessary to provide some means to protect against the high currents and voltages resulting due to so-called Q-multiplication whenever the lamp load is removed or otherwise fails to constitute a proper load for the LC circuit.
In U.S. Pat. No. 4,370,600 to Zansky, circuit protection is provided by way of providing to the LC circuit an alternative load in the form of a voltage-clamping means; which voltage-clamping means acts to load the LC circuit during any period when the lamp does not constitute a proper load therefor.
The voltage-clamping is accomplished by rectifying the Q-multiplied voltage output of the LC circuit and by applying the resulting DC output to the inverter""s DC power source.
However, during any period when voltage-clamping does occur, a relatively large amount of power circulates within the electronic ballast means: from the inverter""s output, through the LC circuit, and back into the inverter""s DC power source by way of the voltage-clamping means.
An object of the present invention is that of providing overload-protection means in a tuned inverter.
This as well as other objects, features and advantages of the present invention will become apparent from the following description and claims.
AC power line voltage is rectified and filtered such as to result in a DC voltage whose instantaneous absolute magnitude is equal to that of the power line voltage except for being prevented from ever falling below about half of the peak magnitude of the power line voltage. A half-bridge inverter is powered from this highly rippled DC voltage. The inverter""s output voltage is loaded by way of a series-tuned high-Q LC circuit. Two fluorescent lamps are series-connected across the secondary winding of a transformer whose primary winding is connected across the tank capacitor of the LC circuit.
The magnitude of the voltage present across the tank capacitor is normally limited by the loading represented by the fluorescent lamps. However, with the lamps removed, if not expressly prevented from doing so, the magnitude of the voltage across the tank capacitor will increase to a destructive level due to Q-multiplication. To prevent this from happening, an auxiliary winding on the transformer is used for limiting the magnitude of the voltage across the tank capacitor by rectifying the output from the auxiliary winding and feeding the resulting DC to the inverter""s DC input. As a result, the magnitude of the voltage across the tank capacitor will be limited to a level determined by the instantaneous magnitude of the DC voltage; which level is set such as to result in acceptable lamp starting while at the same time preventing any voltage-limiting from taking place at any time when the lamps are indeed operating.
To limit internal power dissipation during periods when the lamps are not connected, the effective magnitude of the inverter""s output voltage is made to decrease in a time-delayed manner as a function of the magnitude of the current flowing from the auxiliary secondary winding.
The inverter is of the self-oscillating type and control of the effective magnitude of the inverter""s output voltage is effected by controllably heating the ferrite material of one of two saturable current transformers used in the feedback loop.